The present invention relates generally to gas turbine engines, and in particular to a starting system and method that provides reliable light-off for gas turbine engines.
Gas turbine engines represent a class of engines used to generate kinetic and pneumatic energy. They are employed in a variety of applications, from ground-based power generators to aeronautical applications.
Starting a gas turbine requires the engine to be motored to a certain speed such that the fuel-air mixture provided to the combustor is conducive to light-off (i.e., successful ignition of the engine). The initial kinetic energy is typically provided by a dedicated starter motor or a starter-generator to rotate the APU shaft. Following light-off, the engine is accelerated with the aid of the starter-motor to a self-sustaining speed, at which time the starter motor is disengaged.
Successful light-off depends on the engine speed and flow of fuel delivered to the combustor, which must be coordinated to provide an air/fuel mixture at the igniter capable of sustaining combustion. The range of engine speeds at which starting is most likely to occur is referred to as the “light-off window” and typically ranges from 5% to 20% of the rated engine speed. However, accelerating too quickly through the light-off window typically results in failed ignition. This problem is further complicated if the airflow provided to the compressor and/or combustor is unstable. For example, in aircraft applications in which the auxiliary power unit (APU) is started in-flight, factors such as airspeed of the aircraft and/or wind speed may cause instability in the airflow provided to the combustor. As a result, a particular speed pre-selected for light-off may not be feasible or successful.